A significant feature of stage lighting systems is the projection of images by stage lighting instruments. Images are typically formed by passing a light beam through a light pattern generator or "gobo" and projecting the image formed thereby. A typical gobo is made of a sheet of metal which has the desired image cut from the sheet to form an opening for shaping a light beam passing therethrough. A gobo therefore operates as a light stencil, blocking some portions of the light beam and passing other portions. A typical configuration for creating a pattern of light consists of a gobo placed in a projection gate located at a focal plane of a light projector having a light source, a reflector which focuses light rays to the focal plane, and one or more lenses to project an image formed at the focal plane.
Conventional gobos are limited as to the images that can be produced. Since all of the metal elements of a gobo must be supported, there must often be unwanted support members included in the design. As an example, it is impossible to produce a complete ring design because support members must extend from the exterior to support the interior metal.
Many conventional gobos are stamped out of sheet metal, which makes them very inexpensive when produced in large quantities. The gobos are mounted directly in the intense light beams used in stage lighting, and can become hot enough to glow with a cherry red color. The intense heat produced by the light beam is absorbed by the metal gobo and causes it to soften, warp, and therefore distort the desired image. Some gobos include very thin connecting links supporting internal metal portions of the light stencil from the exterior portions. These thin connecting links, however, are particularly susceptible to damage from the heat of the beam, and under some circumstances can burn through completely, such that the image is severely distorted and the gobo destroyed.
In an effort to produce heat-tolerant gobos, some gobos have been made of heat resistant materials, such as stainless steel, using a laser beam to cut out the desired image. However, even gobos made of these materials have a relatively short lifetime and require frequent replacement. Although the laser-cutting process can produce more complex images than can be stamped, these images are usually characterized by a greater quantity of very thin connecting links and other fine details which can be destroyed in the intense heat of the light beam.
Laser cutting processes used to make aluminum gobos and stainless steel gobos utilize a carbon-dioxide laser, which has a characteristic wavelength of 10.6 micrometers and a typical beam diameter of 12-15 millimeters, or about one-half inch. One such process directs the laser beam through a mask and onto the blank metal gobo. The mask must be reflective to the laser wavelength (10.6 um) and have an opening formed therein in the shape of the desired image. Gold is frequently utilized as the preferred material for reflecting this laser wavelength. Because gold is a relatively expensive material, the cost can be justified only when a relatively large number of identical gobos are to be manufactured.
A second process directs the laser beam through one or more lenses to focus the beam to a smaller diameter, and then directs the beam onto a blank gobo which is mounted on a movable support table. The support table is movable in two axes and is controlled to move the blank gobo so that the laser beam cuts the desired image. This process is more economical than the mask process described above if relatively few gobos are to be manufactured, but the process is still relatively slow. Lenses which are suitable for transmitting and focusing the 10.6 micrometer laser beam are made of exotic and expensive materials, such as: zinc selenide (ZnSe), which is transparent to most visible light but is yellow in color; or gallium arsenide (GaAs) or germanium (Ge), which are opaque to visible light.
Regardless of which laser cutting process is used, the resultant metal gobo in all but the simplest designs includes unwanted connecting links supporting interior details of the metal pattern. Metal gobos also require a certain minimum thickness, depending upon the particular metal used, in order to survive the intense heat of a high-powered entertainment lighting instrument. The thickness of the gobo limits the crispness of the projected image, because it is easier to focus an image formed by a thin gobo than it is to focus an image formed by a thick gobo. Further, metal gobos require a corresponding minimum width of connecting links and other details so as to provide sufficient material strength to support itself, which limits the resolution of fine details obtainable with either stamped or laser-cut metal gobos.
Neodymium:yttrium-aluminum-garnet (Nd:YAG) lasers, having a characteristic wavelength of 1.06 micrometers and a typical beam diameter of 0.001 inch or smaller, can cut any metal that can be cut with a carbon-dioxide laser, but at a significantly slower speed. Nd:YAG lasers are seldom used with nonmetal materials because of low absorption rates of such materials for the energy at 1.06 micrometers. Nd:YAG lasers are used to some advantage for processing metals inside glass or quartz enclosures with no harm to the window.
More recent gobos have been fabricated as a layer of light reflective material, such as aluminum, bonded to a surface of a transparent plate such as heat resistant glass. The light reflective layer has an opening which is in the shape of the image. A portion of the light beam passes through the opening to produce a beam having the shape of the image. The reflective layer serves to reflect a portion of the light beam which does not pass through the opening. Glass gobos of this type are very resistant to the intense heat of entertainment lighting beams, and produce images devoid of unwanted support members or connecting links, all portions of the reflective layer being supported by the transparent substrate. Glass gobos are manufactured by a relatively expensive and time-consuming process, which is described in U.S. Pat. No. 4,779,176.
As described in the above noted patent, a layer of positive photo resist material is deposited in the shape of a desired image onto a large, thin sheet of transparent glass. A thin layer of aluminum then is deposited over the glass and the photo resist layer. A multi-layer dielectric coating deposited over the aluminum layer forms a "black mirror", which is a non-reflective surface that absorbs visible light. The glass sheet and the various coatings are then exposed to acetone which dissolves the photo resist and lifts all the layers of material immediately over the photo resist while having no effect on the glass. The acetone etch process produces an opening through the deposited layers which is in the shape of the desired image. The process can be used to produce a plurality of glass gobos on a single sheet, which is then scribed and broken to separate the individual gobos.
The process described above requires a photo mask having the desired image formed therein to facilitate deposition of the photo resist layer, and significant lead times for the manufacture of finished gobos, thereby making the process more suitable for production of relatively large numbers of gobos than for smaller, "made-to-order" production. Although focused-beam carbon-dioxide lasers can be used as described to achieve economic production of laser-cut metal gobos in small quantities, no comparable method of using laser beams on glass gobos has heretofore been employed, because the wavelength of carbon-ioxide lasers is readily absorbed by materials transparent to visible light and will etch, crack, or otherwise destroy a transparent substrate.